This invention concerns machining methods and more particularly methods for precision machining of thin walled cast iron sleeves.
Modern piston engine design has utilized heavy walled cast iron liner sleeves inserted into bores machined into an aluminum block. The cast iron sleeves provide wear resistance and proper sliding fit of the engine pistons within the cylinder bores.
However, the thinner the wall of the liner, the more weight that would be saved, and there also would be a heat transfer improvement if very thin walled liners were used.
Thus, it has been proposed to use cylinder liner sleeves as thin as 0.035 inches thick for optimal advantage.
However, the inside diameter of the liner must be precision machined to close tolerances, and it would be difficult and costly to properly machine very thin walled sleeves.
The conventional technique for machining involves finish machining the inside diameter of a tubular preform, chucking the preform on an expanding mandrel, and then finish machining the outside diameter. The expanding mandrel would inevitably distort a thin walled sleeve leading to an out of round or distorted condition of the liner sleeve.
It is the object of the present invention to provide an improved method for manufacturing engine cylinder liner sleeves and in particular for producing precision machined thin walled liner sleeves.
It is a further object to provide an improved chuck for use in the precision machining of thin walled tubes.
These objects and others which will be understood upon a reading of the following specification and claims are achieved by first finish machining the outside diameter of a tubular blank or preform. The outside diameter of the preform is then secured in a chuck having engagement surfaces which substantially completely encircle and engage the finish machined outside diameter of the preform to securely clamp the same.
Finish machining of the internal diameter is then carried out, with the encircling chuck engagement surfaces providing a sufficiently rigid support to enable very precise machining of the inside diameter even when only a very thin wall remains after the finish machining is completed. A thin walled precision engine cylinder liner sleeve is thereby produced.
The encircling chuck engagement surfaces comprise portions of a circle defined by each jaw of the chuck.
The chuck is preferably of a special two jaw design, each jaw formed with a semicircular engagement surface, one of the jaws driven by an actuator to be movable relative to the other jaw to completely encircle the outside diameter of the preform. The jaws have abutment surfaces which provide a positive stop, positively limiting further relative closing movement of the jaws, at which point the semicircular engagement surfaces form a circle of a diameter slightly undersized in comparison to the finished outside diameter of the preform.
That is, the two semicircular engagement surfaces define a continuous circle of a diameter slightly smaller than the finished outside diameter size of the preform, for example 0.0002 inches smaller than the finished outside diameter of the preform. This compresses the preform to the degree necessary so as to be securely held during the finish machining of the internal diameter, without causing deformation of the sleeve even when most of the wall thickness is machined away to leave only a very thin wall.